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>55.4. Visibility Map</A
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>Each heap relation has a Visibility Map
(VM) to keep track of which pages contain only tuples that are known to be
visible to all active transactions. It's stored
alongside the main relation data in a separate relation fork, named after the
filenode number of the relation, plus a <TT
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>_vm</TT
> suffix. For example,
if the filenode of a relation is 12345, the VM is stored in a file called
<TT
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>12345_vm</TT
>, in the same directory as the main relation file.
Note that indexes do not have VMs.</P
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>The visibility map simply stores one bit per heap page. A set bit means
that all tuples on the page are known to be visible to all transactions.
This means that the page does not contain any tuples that need to be vacuumed;
in future it might also be used to avoid visiting the page for visibility
checks. The map is conservative in the sense that we
make sure that whenever a bit is set, we know the condition is true, but if
a bit is not set, it might or might not be true.</P
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